Ann. Limnol. - Int. J. Lim. 50 (2014) 185–198 Available online at: Ó EDP Sciences, 2014 www.limnology-journal.org DOI: 10.1051/limn/2014012 Seasonal changes in fish assemblages in the River Minho tidal freshwater wetlands, NW of the Iberian Peninsula Micaela Mota1*, Ronaldo Sousa1,2, Ana Bio1, Jorge Arau´jo1, Catarina Braga1 and Carlos Antunes1,3,4 1 Interdisciplinary Centre of Marine and Environmental Research (CIIMAR/CIMAR), University of Porto, Rua dos Bragas 289, P 4050-123 Porto, Portugal 2 CBMA – Centre of Molecular and Environmental Biology, Department of Biology, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal 3 Aquamuseu do Rio Minho, Parque do Castelinho, 4920-290 Vila Nova de Cerveira, Portugal 4 ESG – Gallaecia University School, Largo das Oliveiras, 4920-275 Vila Nova de Cerveira, Portugal Received 14 August 2013; Accepted 9 April 2014 Abstract – The ichthyofauna of the River Minho tidal freshwater wetlands (TFWs) was studied in a semi- enclosed area, between June 2007 and May 2010, to determine temporal patterns of abundance, biomass and species composition. Fish catches, standardized by the number of fyke nets and by the fishing effort, were analysed and related to river flow, water temperature and precipitation. In total, 21 fish species were identified including six non-indigenous species (NIS) which represented 15% of the total captures (yet 43% of the bio- mass). Regarding ecological guilds, 82% were freshwater species (61% of the biomass) which included all NIS, whereas 13% of the catches were catadromous species (31% biomass) corresponding to eels. Only 4% were estuarine species (1% biomass) and together marine estuarine opportunists and anadromous species ac- complished 1% (2 and 6% biomass, respectively). The fish assemblage showed seasonal patterns with a clear distinction between autumn/winter and spring/summer periods. Significant differences in abundance and bio- mass were also detected between autumn/winter of 2007/2008 and 2008/2009, i.e., inter-annual variations, with the latter period with lower catches yet higher dominance of NIS. Besides, higher water temperature had a significant negative effect on the observed number of species, but a positive effect on the abundance and biomass of NIS. More attention has to be given to TFWs and their role in structuring fish assemblages, be- cause this information is vital for a sound management, conservation and restoration of estuarine areas. Key words: abiotic factors / assemblage / ichthyofauna / Minho estuary / non-indigenous species Introduction In recent years, there have been numerous studies on estuarine areas, addressing a multitude of ecological sub- Estuaries are well-recognized areas of high productiv- disciplines and focusing on a great variety of organisms ity that support a diversity of species, being also (Attrill, 2002; Attrill and Rundle, 2002; Sousa et al., responsible for important ecological functions and services 2008a). Most of these studies focused on the lower (Little, 2000; Vasconcelos et al., 2007). However, they estuarine areas, subjected to brackish water conditions, have been subjected to a great variety of human impacts almost ignoring those upstream estuarine areas that still (e.g., pollution, overexploitation, loss of habitat and continue to be influenced by tides but are subjected to climate change), which have caused depletion of numerous freshwater conditions. These tidal freshwater wetlands formerly important species, destroyed or fragmented (TFWs) constitute a particular estuarine habitat where important estuarine habitats, such as seagrasses, oyster characteristic plant and animal species occur, reflecting reefs and marshes, degraded water quality and introduced the unique biological, chemical and physical conditions. several non-indigenous species (NIS) (Kappel, 2005; Lotze Productivity of TFWs is similar to that of salt marshes but et al., 2006). plant and macrofaunal diversity are higher. Additionally, TFWs provide areas with low tidal influence and essential *Corresponding author: [email protected] passages for commercially and conservationally important Article published by EDP Sciences 186 M. Mota et al.: Ann. Limnol. - Int. J. Lim. 50 (2014) 185–198 Fig. 1. Map of the River Minho estuary (Caminha-Valenc¸a) and the study area showing the TFW semi-enclosed sampling location (arrow). migratory fish species and offer nursery conditions or studies were mostly focused on the lower estuary, and refuges from predators for juvenile fish, molluscs and those regarding fish species were centred on few species crustaceans, provide essential feeding and drinking (mostly related with professional fishing) and have not grounds for birds and constitute ideal habitats for considered temporal changes; thus River Minho TFWs numerous other vertebrate and invertebrate species are currently poorly known. The temporal (seasonal and (Sousa et al., 2008a; Barendregt et al., 2009; Kraus and inter-annual) variation of fish species was analysed, with Jones, 2012). species classified according to their origin (endemic, native One of the most important faunal groups colonizing or non-indigenous), and their ecological and feeding estuaries is fishes. In the particular case of TFWs, these guilds. Observations were further related to water tem- areas may be colonized by estuarine and marine species perature, river flow and precipitation, as potentially with physiological capacities to tolerate changes in relevant environmental factors structuring the fish salinity, migratory fish species use TFWs as a passage to assemblage. upstream areas, and true freshwater species that usually colonize upstream estuarine areas (Schuchardt et al., 1993; Roegner et al., 2010). Despite the ecological importance of Materials and methods fish species colonizing TFWs, there are still few data and little knowledge about the major factors driving spatial Study area and temporal fluctuations in their abundance, biomass and species composition, and the contribution of these The River Minho is an international water body. The TFWs to the ichthyofauna of the whole estuary or basin. last 77 km, from its total length of approximately 300 km, Recognizing this gap in knowledge and considering are part of the northwestern boundary between Portugal that TFWs have suffered water and habitat degradation, and Spain (Fig. 1). Its drainage basin covers 17 080 km2, enhanced understanding of the factors that influence and drains to the Atlantic Ocean. The Minho estuary, species distribution are needed. In theory, this information located on the northwest coast of the Iberian Peninsula will then assist better management, restoration or pro- (IP), has a total area of 23 km2, a length of about 40 km tection of natural resources and ecosystems (Buisson et al., and a maximum width of just over 2 km near the mouth. 2008). It is a mesotidal estuary with a mean tidal range of 2 m. Therefore, the aim of this study was to increase the Salt intrusion extends on average 11.3 km and up to existing knowledge about the ichthyofauna from Iberian 16.8 km inland during higher spring tides (September). estuarine TFWs, using the River Minho as a case study. In Water movements are highly dynamic and the direction of recent years, several studies have investigated the fauna of river water flow is strongly dependent on the freshwater the Minho estuary, namely macroinvertebrates (Sousa volume. et al., 2008b), epibenthos (Costa-Dias et al., 2010) and Rainfall in the River Minho basin exhibits strong fishes (Antunes and Rodrigues, 2004). However, these seasonal and inter-annual variability which is also M. Mota et al.: Ann. Limnol. - Int. J. Lim. 50 (2014) 185–198 187 400 200 ) 300 150 3 dam 4 200 100 precipitation (mm) precipitation river flow(10 river 100 50 precipitation river flow 0 0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Fig. 2. Two-decades (1990–2009) discharges and precipitation patterns of the River Minho. reflected in high variability in the discharge regime (Fig. 2). between samplings, mean¡SE, n=141). Fish total weight River flow increases usually in autumn or early winter, was measured in situ to the nearest 1 g using a digital Kern with a gradual decline during late spring and summer, a 440-53N (maximum 6000 g) scale or to the nearest 0.01 g typical pattern for Iberian rivers. The annual mean using a digital Sartorius (maximum 420 g) scale, depend- freshwater flow is 300 m3.sx1 (Ferreira et al., 2003). ing on the fish size. Every fish was identified to the species The study was conducted in a relatively shallow (depths level. ranging from 1.5 to 3.0 m), semi-enclosed area located Water temperature and salinity were measured every 14 km upstream from the mouth of the river (Fig. 1) that time the fyke nets content were collected (see above), using suffers slight saline influence (<1), especially during a multiparametrical sea gauge YSI 6820. higher spring tides in the summer/early autumn. This area Discharge data for the Minho River were supplied by covers approximately 0.05 km2 and it is greatly hetero- the Confederacio´n Hidrogra´fica del Min˜ o-Sil (Ministerio geneous, with parts covered with submerged vegetation, de Medio Ambiente y Medio Rural y Marino – Spain parts with fine sediments and others with clean sands. This Government), concerning the Frieira dam discharge and heterogeneity at a small scale makes this semi-enclosed are presented in cubic decameters (dam3). area particularly suitable for the study of temporal Precipitation data were obtained from the available changes in abundance, biomass and diversity of fish internet data set of the Monte Aloia (Galicia, Spain) species. meteorological station. Sampling strategy Data analysis From June 2007 to May 2010, fyke nets were used to Temporal variation was assessed in terms of abun- capture fish. These nets have been successfully deployed in dance, biomass, diversity and richness, distinguishing many river conditions, capturing juveniles and adults of endemic, native and NIS. the large-bodied species and most of the smaller-bodied Each fish species was assigned to one of the following species (Humphries et al., 2008; Sousa et al., 2013).